The evolution of the microprocessor has afforded increasingly complex control systems for a number of vehicle applications. However, even the most sophisticated control system employing the fastest microprocessor available must rely on its sensors and actuators to effect control of the system. Thus, more sophisticated control systems rely on repeatable, reliable, and accurate information to execute a control strategy having those same characteristics.
As the sophistication of the vehicle control system increases, the reliance of the vehicle operator on the performance of that system increases accordingly. Thus, vehicle operators expect the vehicle control system to alert them to any operating anomalies, while compensating for those anomalies until it is convenient to have the vehicle repaired. In many of today's vehicles, the operator may be reminded that a door is ajar, the trunk is ajar, a safety belt is unfastened, the engine oil should be changed, or the headlights have been left on unintentionally, among myriad other vehicle operating conditions. Other, more serious operating anomalies, such as an anti-lock brake system failure, engine cooling system failure, or engine lubrication system failure require prompt operator attention to prevent damage to the system, or to avert a potential safety hazard to the operator. The ability of the vehicle control system to alert the operator to any of the above enumerated operating anomalies is dependent upon appropriate system sensors.
A sensor malfunction may lead to an unneeded visit to a service professional to diagnose and correct a non-existent problem. After repetitive false alarms, a vehicle operator may disregard an indication of a hazardous operating condition when one actually occurs. Similarly, a sensor which does not detect a faulty or inoperative vehicle system may lead to unnecessarily extensive repairs, or a potential safety hazard. Thus, the integrity of the system, and the resulting confidence of the operator in the system, depend upon the integrity of the sensors.
An often overlooked vehicle component which is essential to proper vehicle operation is the pneumatic tire. An improperly inflated vehicle tire may be manifested as reduced efficiency in gas mileage, reduced performance in ride and handling, reduced performance in vehicle braking, premature wearing of the tire, or a more serious flat tire or blow-out, among other potential diminutions in vehicle performance or operating safety. Tire pressure is generally measured when the tires are originally inflated or when a tire is noticeably under-inflated. Unfortunately, noticeable under-inflation is typically at an inflation pressure significantly lower than the optimal operating inflation pressure. Thus, the tire has already been operated while improperly inflated, leading to any of the number of problems noted above.
A number of prior art devices have attempted to solve the problem of improperly inflated vehicle tires by providing an automatic tire pressure monitoring system. Typically, these systems monitor tire parameters, such as temperature and pressure, and provide an indication to the vehicle operator if any of the vehicle tires are improperly inflated or a potential safety hazard exists due to severe underinflation (which may be a flat tire or a blow-out). Many of the prior art systems provide a remote sensor within each of the tires in addition to a transmitter for transmitting a signal to a centrally located receiving unit. However, each of the prior art systems has succumbed to at least one of the many varied challenges imposed upon a sensor subjected to the incredibly harsh operating environment of a vehicle tire.
A typical vehicle operating environment is not particularly amenable to the transmission and reception of digital or analog signals. Since the transmitter is often located entirely within a vehicle tire, powered by a battery separate from the vehicle battery, the receiver must be especially sensitive to the detection of relatively weak signals present in an electrically noisy milieu. Furthermore, the various electrically conductive components found on a typical vehicle may facilitate electrical communication but tend to hinder radio wave transmissions. For example, while a steel-belted tire resists penetration by sharp objects, a signal transmitted from within the tire may be severely attenuated by those very same steel belts.
Another problem associated with a transmitter disposed upon or within a vehicle tire is that rotation of a transmitting antenna induces a Doppler frequency shift in the transmitted signal if the antenna is not oriented with its center of mass along the axis of rotation of the vehicle tire. Thus, systems which utilize the valve stem of a tire as the transmitting antenna must provide complex detection circuitry to compensate for the Doppler shift. Alternatively, such systems may dramatically increase the redundancy of transmissions since much of the information will be filtered out as noise.
As with other vehicle components, a vehicle tire sensor must tolerate a wide range of temperature variations. Furthermore, a vehicle tire sensor must perform reliably under significant shock impacts and vibrations, a varying centrifugal force, and a constant applied pressure. Many of the currently available pressure sensors experience inelastic deformation of their microstructures which is manifested as a significant drift when subjected to a constant load. For example, one such sensor may drift almost 25% from its original reading when subjected to a constant load for only one (1) week.
Yet another drawback of prior art pressure sensors as applied to a vehicle tire monitoring system is the significant amount of hysteresis and nonlinear operation over their usable range. A hysteresis of up to 20% between increasing readings and decreasing readings is not uncommon. Of course, hysteresis effects and nonlinearities may be accommodated by using these sensors in conjunction with appropriate processing circuitry which typically consists of a microprocessor. Although microprocessors afford significant flexibility in programming and control functions, they consume a relatively large amount of power and generate a proportionately large amount of heat which must be dissipated. The large power consumption is undesirable for systems which are powered by a small battery.
If a sensor is externally mounted, or has an externally mounted antenna, it is subjected to even greater requirements due to exposure to the elements including water, mud, snow, ice, and the like. Thus, an externally mounted component must resist contamination by dirt and debris while also functioning reliably under conditions adverse to radio frequency (RF) transmissions. For example, an externally mounted antenna should transmit a detectable signal while immersed in water, snow, or mud, especially for commercial applications which frequently encounter such unfavorable conditions.
Another problem associated with prior art devices is in the packaging of the tire sensing apparatus. Those systems employing complex circuitry which may include a microprocessor typically require a large battery to provide sufficient power. For those systems which utilize tire sensing apparatus disposed entirely within the vehicle tires, the impact of the added weight and complexity of the system on vehicle efficiency, performance, and maintenance is critically assessed. Even a device having a relatively insignificant mass requires a counter-weight for acceptable performance at typical tire rotational speeds. The size and weight of some prior art devices makes them impractical for many vehicle applications, such as automotive applications.
Since the energy storage density of commonly available electrochemical storage devices does not facilitate prolonged periods of power consumption, the maintenance costs associated with battery replacement makes many prior art systems unduly expensive or impractical. If the transmitting unit, including the battery, is entirely disposed within the vehicle tire, battery replacement requires dismounting of the vehicle tire from the wheel. Furthermore, dismounting of the tire is also required to reprogram the monitoring device to recognize a different tire location, such as when tires are rotated. Dismounting may also be required to determine if the monitoring device is operating correctly. The inconvenience and increased cost associated with such a device often outweighs the benefits associated with proper inflation of the vehicle tires.
To increase battery life, some prior art devices transmit a signal only when improper inflation is detected. Other devices use the centrifugal force created by rotation of the tire to activate the tire monitoring device. These types of systems allow a window of opportunity for undetected damaging operation to occur. For example, a centrifugally-actuated system would not alert an operator to a flat tire until the vehicle reached a predetermined operating speed. This may result in additional damage to the flat tire. A system which only detects a flat tire would allow operation of an improperly inflated tire which was not yet flat but which could lead to premature tire wear requiring replacement.
To detect the relatively weak signals generated by a typical tire monitoring device, some prior art systems transmit a digital code representing at least one parameter of the tire. For example, U.S. Pat. No. 5,231,872 to Bowler et al. transmits a code representing an instantaneous signal value which includes an identification code and a plurality of receiver resynchronization codes for resynchronizing the central receiving unit. Such systems are unnecessarily complex and result in higher manufacturing, assembly, and maintenance costs. Furthermore, transmission of an instantaneous signal may lead to false alarms since tire inflation pressure may vary significantly during normal operation. For example, tire inflation may vary up to 7 psi between a tire exposed to direct sunlight and a tire on the same axle which is in the shade.